NIRMOS (Near-Infrared Multiple Object Spectrograph) is a 0.9 to 2.5 μm imager/spectrograph concept proposed for the
Giant Magellan Telescope<sup>1</sup> (GMT). Near-infrared observations will play a central role in the ELT era, allowing us to
trace the birth and evolution of galaxies through the era of peak star formation. NIRMOS' large field of view, 6.5′ by
6.5′, will be unique among imaging spectrographs developed for ELTs. NIRMOS will operate in Las Campanas' superb
natural seeing and is also designed to take advantage of GMT's ground-layer adaptive optics system. We describe
NIRMOS' high-performance optical and mechanical design.
The f/5 instrumentation suite for the Clay telescope was developed to provide the Magellan Consortium observer community with wide field optical imaging and multislit NIR spectroscopy capability. The instrument suite consists of several major subsystems including two focal plane instruments. These instruments are Megacam and MMIRS. Megacam is a panoramic, square format CCD mosaic imager, 0.4° on a side. It is instrumented with a full set of Sloan filters. MMIRS is a multislit NIR spectrograph that operates in Y through K band and has long slit and imaging capability as well. These two instruments can operate both at Magellan and the MMT. Megacam requires a wide field refractive corrector and a Topbox to support shutter and filter selection functions, as well as to perform wavefront sensing for primary mirror figure correction. Both the corrector and Topbox designs were modeled on previous designs for MMT, however features of the Magellan telescope required considerable revision of these designs. In this paper we discuss the optomechanical, electrical, software and structural design of these subsystems, as well as operational considerations that attended delivery of the instrument suite to first light.
The Giant Magellan Telescope (GMT) is a 25.4-m optical/infrared telescope constructed from seven 8.4-m primary
mirror segments. The collecting area is equivalent to a 21.6-m filled aperture. The instrument development program was
formalized about two years ago with the initiation of 14-month conceptual design studies for six candidate instruments.
These studies were completed at the end of 2011 with a design review for each. In addition, a feasibility study was
performed for a fiber-feed facility that will direct the light from targets distributed across GMT's full 20 arcmin field of
view simultaneously to three spectrographs. We briefly describe the features and science goals for these instruments, and
the process used to select those instruments that will be funded for fabrication first. Detailed reports for most of these
instruments are presented separately at this meeting.
The Giant Magellan Telescope (GMT) is a 24.5m diameter optical/infrared telescope. Its seven 8.4m primary mirrors
give it a collecting area equivalent to a 21.4m filled aperture. The ten GMT partners are constructing the telescope at the
Las Campanas Observatory in Chile with first light planned for the end of 2018. In this paper, we describe the plans for
the first-generation focal plane instrumentation for the telescope. The GMTO Corporation has solicited studies for
instruments capable of carrying out the broad range of objectives outlined in the GMT Science Case. Six instruments
have been selected for 14 month long conceptual design studies. We briefly describe the features of these instruments
and give examples of the major science questions that they can address.
The Giant Magellan Telescope, with seven 8.4 meter primary mirrors, is taking shape as one of the most powerful
telescopes of the next generation. We describe a conceptual design for a powerful 0.85 to 2.50 μm imaging
spectrograph that addresses a 7' by 7' field of view for imaging and a 5' by 7' field of view for spectroscopy at the
GMT's f/8 Gregorian focus. The all-refractive optical design presses the limits of available lens blank diameters, but
delivers excellent images (~0.15" 80% encircled energy) with just four collimator elements and five camera elements.
The collimated beam diameter is 300 mm, and the detector is a 6K by 10K array. The spectrograph will use
interchangeable slit masks, and an assortment of VPH and conventional surface relief gratings. Each of the entire J, H,
or K bands can be observed with a resolution of 3000. The scientific potential of ground layer adaptive optics (GLAO)
using a constellation of sodium laser guide stars appears to be very high in the near infrared. Simulations suggest that
0.2" FWHM images may be achieved across the entire 7' by 7' field of view of the spectrograph. We describe the
design of the GLAO system with a versatile opto-mechanical design that allows rapid changeover between GLAO and
We have developed practical, high performance flexure mounts for large astronomical lenses in the Binospec
spectrograph. Flexure mounts are an attractive alternative to the widely used elastomeric lens mounts when high axial
stiffness is a priority and coupling fluids are incompatible with elastomers. We describe coupling fluid seals for the
In March and April 2003, the Chandra X-ray Observatory carried out a
series of 126 short observations (5 ksec each) covering a continuous
area of the Bootes constellation to construct a large area shallow
X-ray survey. These observations were carried out as collaboration of
Guest Observer (C. Jones PI) and Guaranteed Time Observer (S. Murray
PI) programs. We present here, in Paper I, an initial analysis of the
survey data and the source detection process, showing the sky
coverage, exposure map, and some of the collective properties of the
resulting catalog of sources. The Bo\"otes area was selected to
overlap a well studied region where optical, and radio data, to
sufficient depth, have already been obtained making the identification
of candidate counterparts straight forward. In 5 ksec, we reach a
limiting flux of ≈10<sup>-3</sup>ct s<sup>-1</sup> (corresponding to ≈10<sup>-14</sup> erg cm<sup>-2</sup>s<sup>-1</sup>0.5-7.0 keV). We examine the spatial distribution of the sources in this <sup>~</sup>9.3 square degree survey region using several techniques to search for evidence of cosmic variance in the X-ray source density on scales as small as the ACIS-I field of view
(<sup>~</sup>16x16 arc minutes). With follow up optical spectroscopy using the MMT/Hectospec, we can obtain spectroscopic redshifts for about 1/3 - 1./2 of the X-ray sources, which can be used to look for evidence of large scale structures traced by AGN associated with the cosmic web.
We present the preliminary design for the MMT and Magellan Infrared
Spectrograph (MMIRS). MMIRS is a fully refractive imager and multi-object spectrograph that uses a 2048x2048 pixel Hawaii2 HgCdTe array. It offers a 7'x7' imaging field of view and a 4'x7' field of view for multi-object spectroscopy. Dispersion is provided by a set of 5 grisms providing R=3000 at J, H, or K, or R=1300 in J+H or H+K.
At its f/5 focus, the 6.5 meter converted MMT uses a refractive corrector to produce excellent images over a field of view as large as 1° in diameter. We describe the construction challenges we encountered and the lessons we learned mounting the ~30 inch diameter lenses for this wide-field corrector. The corrector was completed in May 2002, was commissioned in May and June of 2003, and is now in regular use at the MMT for spectroscopic and imaging observations.
In 2003, the converted MMT’s wide-field f/5 focus was commissioned. A 1.7-m diameter secondary and a large refractive corrector offer a 1° diameter field of view for spectroscopy and a 0.5° diameter field of view for imaging. Stellar images during excellent seeing are smaller than 0.5" FWHM across the spectroscopic field of view, and smaller than 0.4" across the imaging field of view. Three wide-field f/5 instruments are now in routine operation: Hectospec (an R~1000 optical spectrograph fed by 300 robotically-positioned optical fibers), Hectochelle (an R~40,000 optical spectrograph fed by the same fibers), and Megacam (a 340 megapixel, 36 CCD optical imager covering a 25' by 25' format).
The 6.5m Multiple Mirror Telescope Observatory (MMTO) installed a new f/5 secondary system in April 2003. We describe the design and performance of the mirror cell and supports for the 1.7 m diameter Zerodur mirror. Pneumatic actuators divided into one lateral and three axial zones support this 318 kg mirror. The control feedback for the high bandwidth pressure transducers for these four zones is obtained from six load cells attached to rigid positioning rods. The mirror cell includes thermal control, force limiters, passive supports, installation and handling, and alignment metrology. Optical test results are described and compared to the original design specifications.
The Giant Magellan Telescope (GMT) is a joint project of a consortium of universities and research institutions to build and operate a 21.5-m equivalent aperture astronomical telescope for use at visible and IR wavelengths. This paper briefly summarizes the science goals for the project and provides an overview of the preliminary telescope and enclosure concepts and site test program. The telescope is a Gregorian design with a fast, f/0.7, primary mirror that allows a compact and stiff mount structure. The 25.3-meter diameter primary mirror consists of six off-axis 8.4-meter circular mirrors arranged in a hexagon around a center 8.4-meter mirror. The Gregorian secondary mirror is adaptive allowing two-mirror, wide-field adaptive optics. Several corrector designs have been studied for wide-field applications and one such design is shown. Instruments being considered for GMT provide a wide range of scientific capabilities. Instruments mount below the primary mirror on an instrument platform. Instrument mounting and servicing provisions are summarized.
Shack-Hartmann wavefront sensors have been commissioned and are now in
routine use at both of the optical foci (f/9 and f/5) of the converted
MMT. Both units are of moderate resolution with 14x14 square
apertures across the pupil for f/5 and 13x13 hexagonal apertures for
f/9. They share a common software interface that fits a set of 19
Zernike polynomials to the wavefront errors. Zernike focus and coma
are corrected by moving the secondary mirror, third order spherical by
a combination of secondary motion and primary bending, and the rest by
primary bending alone. In this paper we will describe the two
wavefront sensor systems and how they have performed thus far.
We have completed a detailed thermal analysis of Binospec, a wide-field, multi-slit spectrograph being developed for the 6.5m MMT. The goals of our analysis were to minimize temperature gradients and thermally-induced deflections and achieve a > 24 hr time constant in the spectrograph optics. We consider the effects of conduction, convection, and radiation with the external environment, and model the consequences of opening a spectrograph to insert new slit masks or filters. We study when internal heat sources balance environmental effects, and the local effects of a hot motor in a spectrograph. We review the results of these thermal analyses and draw general conclusions useful to instrument builders.
We describe our plans to add cross-dispersion and an integral field unit to the Hectochelle spectrograph, a multiobject, fiber-fed echelle spectrograph for the converted MMT. Hectochelle was originally designed without cross-dispersion to be used in a single order or overlapping orders selected by interference filters. The addition of cross-dispersion allows us to trae off multiplex advantage for spectral coverage. Our cross-disperser uses an unusual segmented, zero-deviation prism that is very compact, allowing it to fit into the existing instrument without modification. The planned integral field unit can be used with either Hectochelle or the moderate-dispersion Hectospec bench spectrograph. Both spectrographs were originally designed to be fiber-fed with a robotic fiber positioner as a front end, so adding an integral field capability is a natural enhancement. The integral field unit will use smaller diameter fibers than the robotic fiber positioner (subtending 0".6 vs. 1".5), so that both spectrographs will achieve higher spectral resolution in integral field mode. With the integral field unit Hectochelle will reach a two pixel resolution, R approximately 70,000, and Hectospec will reach R approximately 2000 with its 270 line mm<sup>-1</sup> grating.
Binospec is a binocular optical spectrograph under development for the converted MMT. Binospec addresses two adjacent 8' by
15' fields of view, yielding an effective slit length of
30'. Despite its very wide field of view, Binospec's optics
are compact due to the favorable image scale at the converted MMT's
f/5 Cassegrain focus. However, Binospec's all-refractive collimator
and camera have presented several challenges, including the need for
careful athermalization and high performance optics mounts. In the
course of Binospec's development H.W.E. and D.G.F. developed a new
athermalization technique to maintain image scale, image quality, and
focus over a wide temperature range using thin lenses formed in the
coupling fluid between lens multiplets. Tight specifications for image quality and gravity-induced image motion and defocus lead to tight specifications for displacements of Binospec's optical elements. We describe how Binospec's elastomeric lens mounts have been tuned to attain this level of performance.
We describe the techniques that we have used to mount large optics in three wide-field instruments for the converted MMT: the wide-field corrector uses to provide a 1 degree diameter field at the f/5 focus of the converted MMT, the Hectospec bench spectrograph fed by 300 optical fibers and the wide- field dual-beam Binospec spectrograph. These optics are primarily refractive elements with diameters between 0.2 and 0.8 m that must be mounted from their edges, although we also describe mounts for two large mirrors in the Hectospec bench spectrograph. Both the wide-field corrector and Binospec mounts must perform under varying gravity loads: the corrector is fixed to the converted MMT's primary mirror cell and is tilted from zenith to horizon while Binospec is mounted at the converted MMT's Cassegrain focus. Furthermore, the optics mounts for both instruments must fit within tight space constraints. The Hectospec spectrograph is mounted in the MMT's rotating building and experiences a constant gravity vector. In all cases, the mounts must perform over a wide temperature range, -20 to 20 degrees Celsius, so the issue of differential thermal expansion between the mounts and optics must be carefully considered. As a result, the mounts we discuss include either RTV elastomeric or flexural elements.
Binospec is a wide-field, multi-object optical spectrograph to be used at the f/5 focus of the converted 6.5 m Multiple Mirror Telescope. Its dual beams will address adjacent 8' X 15' fields of view, yielding a total slit length of 30'. Binospec will offer approximately 1 - 6 Angstrom resolution at wavelengths between 0.39 and 1.0 micrometer with a 200 mm collimated beam diameter. Although it is difficult to design an f/5 wide-field collimator, f/5 optics are compact, allowing a small and stiff instrument structure. Binospec uses refractive optics throughout; the collimator contains three lens groups and the camera contains four lens groups. Three aspheric surfaces are used: two in the collimator and one in the camera. A pair of 2048 by 4608 pixel CCD detectors are used for each beam, yielding a sampling of 0.22' per pixel. Binospec's innovative optical design allows excellent image quality. Including the contribution of the MMT optics with the f/5 wide-field corrector, the RMS image diameter at Binospec's focal plane is 18 micrometer (1.3 pixels) averaged over field angles and colors.
Optical fibers with broadband transmission from the UV through the IR have not been available because the silica core material either has OH absorption bands in the IR or UV absorption due to intrinsic structural defects or chlorine. We have developed a new silica core material which can be fabricated into an optical fiber with very good transmission characteristics from 350 nm to 2000 nm. The transmission performance is stable with time because the fiber is not doped with hydrogen.
The Hectochelle will be a fiber-fed, multi-object spectrograph for the post-conversion MMT which will take 255 simultaneous spectra at a resolution of 32,000 - 40,000. The absolute efficiency, including optical fiber losses, is predicted to be 6% - 10%, depending on the position of a line within a diffractive order. In one hour, features with 60 mangstrom should be resolved in m<SUB>R</SUB> equals 18 stars with a signal to noise of 10.
The Hectospec consists of a robotic positioner that will position 300 optical fibers at the f/5 focus of the converted MMT and a bench mounted moderate-dispersion spectrograph. Hectospec will be the first wide-field instrument to be used at the converted MMT and is now under construction at the Smithsonian Astrophysical Observatory. Commissioning at the converted MMT is scheduled for mid 1999, shortly after first light at the f/5 focus. The innovative features of the instrument are described, emphasizing recent developments.
The Hectospec is a moderate dispersion spectrograph fed by 300 optical fibers. Hectospec's pair of five-axis robots will position fibers at the 1 degree diameter f/5 focus of the converted MMT, allowing efficient multi-object spectroscopy. We discuss algorithms that we have developed to match the optical fibers to celestial objects and then to compute the appropriate sequence of robotic positioner moves to reconfigure the fibers between successive observations. Both algorithms require essentially no user interaction, consume only modest computer resources and allow effective deployment of the Hectospec's 300 fibers. The target-to-fiber matching algorithm is a recursive procedure which allows simultaneous optimization of the multiple observations that are required to complete a large survey. The robotic motion sequence algorithm allows the two Hectospec robots to work together efficiently to move fibers directly between observing configurations.
Operated by the Multiple Mirror Telescope Observatory (MMTO), the multiple mirror telescope (MMT) is funded jointly by the Smithsonian Institution (SAO) and the University of Arizona (UA). The two organizations equally share observing time on the telescope. The MMT was dedicated in May 1979, and is located on the summit of Mt. Hopkins (at an altitude of 2.6 km), 64 km south of Tucson, Arizona, at the Smithsonian Institution's Fred Lawrence Whipple Observatory (FLWO). As a result of advances in the technology at the Steward Observatory Mirror Laboratory for the casting of large and fast borosilicate honeycomb astronomical primary mirrors, in 1987 it was decided to convert the MMT from its six 1.8 m mirror array (effective aperture of 4.5 m) to a single 6.5 m diameter primary mirror telescope. This conversion will more than double the light gathering capacity, and will by design, increase the angular field of view by a factor of 15. Because the site is already developed and the existing building and mount will be used with some modification, the conversion will be accomplished for only about $20 million. During 1995, several major technical milestones were reached: (1) the existing building was modified, (2) the major steel telescope structures were fabricated, and (3) the mirror blank was diamond wheel ground (generated). All major mechanical hardware required to affect the conversion is now nearly in hand. Once the primary mirror is polished and lab-tested on its support system, the six-mirror MMT will be taken out of service and the conversion process begun. We anticipate that a 6 - 12 month period will be required to rebuild the telescope, install its optics and achieve f/9 first light, now projected to occur in early 1998. The f/5.4 and f/15 implementation will then follow. We provide a qualitative and brief update of project progress.
The conversion of the Multiple Mirror Telescope from six 1.8 m primary mirrors to a single 6.5 m primary will significantly increase its capability for imaging. The f/5 configuration will provide a corrected field of view for imaging that is flat and 30 arcminutes in diameter. The image quality in the absence of atmospheric seeing is 0'.1 over the full field. We are currently designing a camera system to take advantage of this large field. The proposed direct imaging system will be located at the Cassegrain focus of the telescope, behind a three-element refractive corrector. We will use an array of 8 X 4 three-edge-buttable CCDs, each with 2048 X 4096 pixels and two output amplifiers. This will provide a field of view of 24' X 24'. With a new packaging scheme we will reduce the gap along the readout edge to a few millimeters. The pixel size is 15 microns, or 0'.09, well sampling the point-spread- function. In many applications it will be possible to bin the pixels, thus reducing the amount of data (500 Mb per read at full resolution). The back-illuminated CCDs will be thinned and anti- reflection coated to provide high quantum efficiency from 320 to 1000 nm. The camera system will be useful for many studies requiring both a large collecting area and large area coverage on the sky. Planned projects include redshift and photometric surveys of faint galaxies, searches for high-redshift quasars and searches for objects in the outer solar system.
The f/5 focus of the converted Multiple Mirror Telescope (MMT) has been designed for optical fiber spectroscopy over a 1 degree(s) diameter field as well as superlative imaging over a 0.5 degree(s) diameter field. The secondary mirror required for this purpose is nearly as large as one of the existing MMT primary mirrors (1.7 m). To attain the specified performance of the telescope support structure, this secondary must be made quite light (approximately 300 kg), without excessively compromising its stiffness. Our design calls for removing excess weight by machining hexagonal cells into a near zero-expansion materials such as ULE or Zerodur. The completed blank will be approximately 20 cm thick at the center, tapering to approximately 13 cm at the edge. The support of a secondary of this size must be considered as part of the blank design. We describe two possible axial support techniques: a vacuum support or a multiple actuator support similar to that adopted for the primary mirror of the converted MMT. Tangent flexures at the edge of the blank appear to be the most attractive radial support option.
We describe a multi-object spectrograph that is currently under design for the f/5 focus of the converted Multiple Mirror Telescope. The f/5 Cassegrain focus will use a three element refractive corrector to produce a 1 degree(s) diameter field of view well matched to multi-object spectroscopy with optical fibers. The optical fibers will be mounted on magnetic buttons and positioned with two high speed robots, using the general techniques pioneered by the AUTOFIB instrument. Our goal is to position the 300 fibers in 300 seconds. The spectrograph optics will be bench mounted on the observing floor, and will be fed by fibers that are 10 meters in length. The spectrograph will have a 250 mm diameter collimated beam to allow a resolution of 4 to 8 $angstrom FWHM over approximately an octave of spectrum between 3400 $angstrom and 1 micrometers . It is our intention to complete the Hectospec before the commissioning of the f/5 focus in late 1996.
We describe an approach for mounting the approximately 0.8 m diameter optical elements in a refractive corrector for the Multiple Mirror Telescope (MMT). The optical elements are mounted on discrete pads of RTV rubber to an Invar/carbon steel cell. Following the conversion of the MMT to use a single 6.5 m primary mirror, the corrector will provide up to a 1 degree(s) diameter field-of-view for multi-object spectroscopy with optical fibers or for wide- field imaging.
The project to enlarge the Multiple Mirror Telescope (MMT) to a 6.5 m single primary mirror telescope is described. The goal is to provide a telescope which is competitive with the existing MMT in tracking and pointing performance (0.2 and 1.0 arcseconds, respectively) but has more than twice the light gathering power and 15 times the angular field of view. The existing mount and building will be used with minor modifications so that the cost of the project is relatively modest. Casting of the 6.5 m mirror is scheculed in early 1991 and first light in late 1993.
An automated fiber optics manipulator that allows the acquisition of up to 10 spectra simultaneously with an existing long slit spectrograph has been placed in operation at the Michigan-Dartmouth-MIT Observatory. Pickoff optics that view mirrors at the fiber probe tips allow visual alignment of the probes on the target objects. This feature eliminates the requirement for subarcsecond astrometry, careful focal plane calibration and highly precise actuator motion.